Verb stem mutations question

[tectonic-drift.livejournal.com]

I read about the following pattern:

If the letter Г, з, or Д precedes the stem vowel, then the consonant mutates to a ж. In a first conjugation verb, the mutation occurs in all forms, but in 2nd conjugation verbs the mutation is in 1st person singular only.

However, the verb обедать does not follow the pattern, where as видеть does follow the pattern. Is обедать just an exception or am I missing something?